1. Field of the Invention
The present invention generally relates to optical plates, and more particularly to a prism sheet for use in, for example, a liquid crystal display device.
2. Discussion of the Related Art
FIG. 3 represents a typical liquid crystal display (LCD) device 10. The LCD device 10 includes a backlight module 11 and an LCD panel 12 disposed above the backlight module 11. The backlight module 11 includes a housing 111 and a plurality of lamps 112 for emitting light disposed above a base of the housing 111. The backlight module 11 further includes a light diffusion plate 113 and a prism sheet 114 stacked on the top of the housing 111 in that order. An interior of the housing 111 is configured for reflecting light from the lamps 112 towards the light diffusion plate 113. The light diffusion plate 113 includes a plurality of dispersion particles. The dispersion particles are configured for scattering light, and thereby enhancing the uniformity of the received light emitting from the light diffusion plate 113.
Referring to FIG. 4, the prism sheet 114 includes a transparent base 1141 and a plurality of V-shaped elongated protrusion 1142 formed on the transparent base 1141. Each V-shaped elongated protrusion 1142 extends along a direction parallel to one edge of the prism sheet 114. The plurality of V-shaped protrusions 1142 are regularly and periodically arranged parallel to each other. In use, light emitted from the lamps 112 enters the prism sheet 114 after being scattered in the diffusion plate 113. The light is refracted by the V-shaped protrusions 1142 of the prism sheet 114 and is thereby concentrated, so that a brightness of light illumination is increased. Finally, the light propagates into the LCD panel 12 disposed above the prism sheet 114.
However, it is prone to occur moire pattern interference on the LCD panel 15 due to the V-shaped structures 102 being aligned with the LCD pixels. Referring to FIG. 5, in order to decrease the moire pattern interference, another typical prism sheet 20 is provided. The prism sheet 20 includes a transparent base 201 and a plurality of V-shaped protrusion 202 formed on the transparent base 201. Each V-shaped protrusion 202 extends along an irregular curve line. The V-shaped protrusions 202 have a same curvature at a corresponding portion. Since each of the V-shaped protrusion 202 extends along the irregular curve line, the V-shaped protrusions 202 are aligned obliquely with the LCD pixels, and thus the moire pattern interference will be not occurred.
Furthermore, it is difficult to manufacture the prism sheet 20 by injection molding method, because it is difficult to prepare a mold having a surface defining a plurality of curved V-shaped depressions according to the V-shaped protrusions 202. Typically, in a cutting process of making the mold, a cutter having V-shaped blade is driven along different directions according to a predetermined pattern. In other words, the cutter should be driven encircle related to different points according to the curvatures of the V-shaped depression of the predetermined pattern. Because the curved V-shaped depressions of the surface of the mold are very small in size, it is costs time to form the curved V-shaped depressions in the surface of the mold. In addition, it is difficult to keep the precision of the curved V-shaped depression. Therefore, design and manufacturing costs of the mold is increased, and the prism sheet 20 may have a low optical performance.
What is needed, therefore, is a new prism sheet and a liquid crystal display device using the prism sheet that can overcome the above-mentioned shortcomings.